A method for calculating the constructive, cooperating-point dependent maximum and minimum adjusting speeds in a continuously variable transmission with electrohydraulic drive.
A continuously variable transmission usually consists, among others, of a starting unit, a forward/reverse drive unit, an intermediate shaft, a differential, hydraulic and electronic control devices and a variator. The variator customarily comprises a primary and a secondary pulley also called primary and secondary side, both pulley being formed by cone pulleys disposed in pairs and being provided with a torque-transmitting belt-drive element which rotates between the two cone pulley pairs.
In a transmission of that kind the actual multiplication is defined by the running radius of the belt-drive element which in turn is function of the axial position of the cone pulleys.
Continuously variable transmission s generally have in comparison with standard mechanical transmissions one more degree of freedom determined by principle, since added to the selection of the multiplication step to be adjusted, it is here possible also to predetermine and influence the adjustment speed at which the multiplication is transmitted form one operating point to another.
In continuously variable transmissions having a continuously variable element (belt, chain) as torque-transmitting part, there results from the structural design that in the multiplication change the cone pulley pairs of primary and secondary side of the variator are alternatively and complementarily pushed apart and pushed together by corresponding control elements, which causes a change of the running radius of the continuously variable element upon the cone pulleys and thus a change of the multiplication between primary and secondary sides.
The variator usually is hydraulically controlled. The axial displacement of the cone pulleys means here a change of volume which, since the adjustment progresses controlled by force or pressure, has to be compensated in the cone pulley pair concerned by the control hydraulics via adequate changes of volume flow.
The change of volume flow to be adjusted by the electrohydraulic control depends here directly on the actual adjustment speed of the cone pulley pairs.
Since the control hydraulics as a rule is supplied via an engine-dependent pump with structurally determined maximum volume flow, there necessarily results also a constructively stationary limit for the implementable adjustment dynamics of the variator. The variator can be adjusted only as quickly as allowed by the available oil volume flow in the interplay with other regulation and control loops or consumers.
In the constructional design of the supply pump an essential part is played, together with the assurance of the necessary oil volume flow, aspects like noise and efficiency both of which as a rule act negatively as the size of the pump increases. This results in that for the constructional design of a pump a compromise is implemented between the different criteria which compromise based on the operating point and the individual criteria constitutes only a less than optimal solution.
Based on the adaptable adjustment speeds of the variator, means that there will always exist operating conditions where adjustment gradients higher than allowed by the actual availability of the oil volume flow would be theoretically possible.
Said operating conditions are specially critical for a superposed control device, since the control without the transmission medium oil has no prevalence on the behavior of the variator and thus on the multiplication adjustment. The consequence is instabilities which can produce the destruction of the mechanics of the transmission.
One other aspect is formed by the limitations on the variator determined by the design (strength of the parts, limiting values for control pressures) which, to prevent damage or even destruction of the transmission mechanics, likewise have to be taken into consideration at every moment.
A simple possible implementation would be to preset for the admissible adjustment gradient empirical limiting values which are far enough removed from the critical values. The disadvantage here is that the possible adjustment potential in this case cannot be utilized to the required extent. Besides, a generalization regarding safety in all operating conditions is hardly possible.
This invention is therefore based on the problem of indicating, departing from the cited prior art, a method for calculating and taking into consideration the constructive, operation-point dependent maximum and minimum adjusting speeds in the multiplication control of a continuously variable transmission with electrohydraulic drive.
It is accordingly proposed steadily to calculate in every operating condition, via a physical mathematical pattern, the actual limiting values for the maximum possible adjustment gradients. Here are taken into account the special marginal conditions of oil supply (volume flow limiting values) and geometric ratios on the variator determined by the design. The calculations are based on a mathematical physical pattern. According to a selection method for establishing the adjusting speed limits relevant for the superposed adjustment control, these values are used in a control loop to adjust a predetermined multiplication specified value.
Here is used a control loop structure such as described in the Applicant""s DE 196 06 311 A1. Such control loop structures combine a physical-mathematically pattern-based linearization of the controlled system by a correction member with a linear proportional plus integral plus derivative (PID) controller. The correcting variable of the PID controller is directly interpreted as standard for the adjustment gradient to be set.